Dual connection power line parameter analysis method and system

ABSTRACT

A method and apparatus is disclosed for determining the power line parameters of a system. Specifically, there is provided a method comprising perturbing a voltage waveform through a first connection, measuring a characteristic of the perturbation through a second connection, and calculating a line impedance based on the characteristic of the perturbation.

BACKGROUND

The present technique relates generally to the field of electricaldistribution. Specifically, the invention relates to techniques fordetermining the impedance parameters of electrical power, fordetermining incident energy, for determining a flash protectionboundary, and for determining a level of personal protective equipment(“PPE”) that may be required or advisable based upon the availableenergy and similar considerations.

Systems that distribute electrical power for residential, commercial,and industrial uses can be complex and widely divergent in design andoperation. Electrical power generated at a power plant may be processedand distributed via substations, transformers, power lines, and soforth, prior to receipt by the end user. The end user may receive thepower over a wide range of voltages, depending on availability, intendeduse, and other factors. In large commercial and industrial operations,the power may be supplied as three phase ac power (e.g., 208 to 690 voltac, and higher). Power distribution and control equipment thenconditions the power and applies it to loads, such as electric motorsand other equipment. In one exemplary approach, collective assemblies ofprotective devices, control devices, switchgear, controllers, and soforth are located in enclosures, sometimes referred to as “motor controlcenters” or “MCCs”. Though the present techniques are discussed in thecontext of MCCs, the techniques may apply to power management systems ingeneral, such as switchboards, switchgear, panelboards, pull boxes,junction boxes, cabinets, other electrical enclosures, and distributioncomponents.

A typical MCC may manage both application of electrical power, as wellas data communication, to the loads, such loads typically includingvarious machines or motors. A variety of components or devices used inthe operation and control of the loads may be disposed within the MCC.Exemplary devices contained within the MCC are motor starters, overloadrelays, circuit breakers, and solid-state motor control devices, such asvariable frequency drives, programmable logic controllers, and so forth.The MCC may also include relay panels, panel boards, feeder-tapelements, and the like. Some or all of the devices may be disposedwithin units sometimes referred to as “buckets” that are mounted withinthe MCC. The MCC itself typically includes a steel enclosure built as afloor mounted assembly of one or more vertical sections containing thebuckets.

The MCC normally contains power buses and wiring that supply power tothe buckets and other components. For example, the MCC may house ahorizontal common power bus that branches to vertical power buses withinthe MCC. The vertical power buses, known as bus bars, then extend thecommon power supply to the individual buckets. Other large powerdistribution equipment and enclosures typically follow a somewhatsimilar construction, with bus bars routing power to locations ofequipment within the enclosures.

To electrically couple the buckets to the vertical bus, and to simplifyinstallation and removal, the buckets may comprise electrical connectorsor clips, known as stabs. To make the power connection, the stabs engage(i.e., clamp onto) the bus bars. For three phase power, there may be atleast three stabs per bucket to accommodate the three bus bars for theincoming power. It should be noted that though three phase ac power isprimarily discussed herein, the MCCs may also manage single phase ordual phase ac power, as well as dc power (e.g., 24 volt dc power forsensors, actuators, and data communication). Moreover, in alternateembodiments, the individual buckets may connect directly to thehorizontal common bus by suitable wiring and connections. Similarly, incontexts other than MCCs, the structures described herein will, ofcourse, be adapted to the system, its components, and any enclosuresthat house them.

A problem in the operation of MCCs and other power management systems,such as switchboards and panelboards, is the occurrence of arcing (alsocalled an arc, arc fault, arcing fault, arc flash, arcing flash, etc.)which may be thought of as an electrical conduction or short circuitacross the air between two conductors. Initiation of an arc fault may becaused by a loose connection, build-up of foreign matter such as dust ordirt, insulation failure, or a short-circuit between the two conductors(e.g., a foreign object establishing an unwanted connection betweenphases or from a phase to ground) which causes the arc. Once initiated,arcing faults often typically proceed in a substantially continuousmanner until the power behind the arc fault is turned off. However,arcing faults can also comprise intermittent failures between phases orphase-to-ground. In either case, the result is an intense thermal event(e.g., temperatures up to 35,000° F.) causing melting or vaporization ofconductors, insulation, and neighboring materials.

The energy released during an arcing fault is known as incident energy.Incident energy is measured in energy per unit area, typically Joulesper square centimeter (J/cm²). Arcing faults can cause damage toequipment and facilities and drive up costs due to lost production. Moreimportantly, the intense heat generated by arcing faults has led to theestablishment of standards for personal protective equipment (“PPE”)worn by service personnel when servicing electrical equipment.

There are five levels of PPE numbered from 0 to 4. Whereas, level 0 PPEcomprises merely a long sleeved shirt, long pants, and eye protection,level 4 PPE comprises a shirt, pants, a flame retardant overshirt andoverpants, a flash suit, a hard hat, eye protection, flash suit hood,hearing protection, leather gloves, and leather work shoes. PPE levels1–3 comprise increasing amounts of protective clothing and equipment inincreasing greater amounts between levels 0 and 4. As such, the higherthe PPE level, the more protective clothing or equipment a person willput on (referred to as “donning”) or take off (known as “doffing”)before servicing the equipment. Accordingly, the time to donn and doffthe protective equipment increases as the PPE level increases. Forexample whereas it may take less than a minute to donn or doff level 1PPE, it may take 20 minutes or more to donn or doff level 4 PPE. Thesedonning and doffing times can directly affect productivity. As such, itis advantageous to accurately determine the potential incident energy ofa potential arc flash so that the appropriate level of PPE.

Many other uses and applications exist for information relating toincident energy, and other power line electrical parameters. Theseinclude, but are not limited to, the sizing and design of filters, thecommissioning and design of motor drives and other equipment, themonitoring of power lines and components for degradation and failure,and so forth.

Conventional methods for determining power line parameters and PPElevels rely on approximation techniques or require complex, extensivemodeling of electrical equipment. There is a need in the art forimproved techniques for determining the incident energy. There is aparticular need for a technique that would permit the accuratedetermination and communication of incident energy, the determination offlash protection boundaries, and the determination and communication ofPPE levels that correspond to a particular incident energy.

BRIEF DESCRIPTION

The present invention provides novel techniques for determining theincident energy of a potential arc flash in an electrical device, fordetermining a flash protection boundary, for determining a PPE level,and for communicating such determinations to users and servicepersonnel. The techniques can be used on single-phase and three-phaseapplications with little modification. Moreover, the technique can beimplemented in permanent (i.e., hard-wired) circuitry, or can be part ofportable or even hand-held devices used to determine the incident energyon a periodic or sporadic basis. Still further, the technique may beimplemented in a stand-alone embodiment or in a distributed network.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a diagrammatical representation of a power line impedancemeasurement system in accordance with aspects of the present technique,applied to a single-phase application;

FIG. 2 is a somewhat more detailed view of certain of the circuitry ofthe power line impedance measurement system of FIG. 1;

FIG. 3 is a diagrammatical representation of certain exemplary steps inidentifying power line impedance values based upon the circuitry ofFIGS. 1 and 2;

FIG. 4 is a voltage waveform and switching waveform for a solid stateswitch of the circuitry illustrated in FIG. 2 for causing a voltagedroop and a resonant ring used to identify impedance parameters;

FIG. 5 is a detailed view of an exemplary resonant ring caused in avoltage waveform and used for determine certain of the impedanceparameters in accordance with aspects of the present technique;

FIG. 6 is graphical representation of a voltage waveform similar to thatof FIG. 4, before exemplary filtering of sampled data;

FIG. 7 is a graphical representation of the waveform of FIG. 6 followinghigh pass filtering of sampled data to flatten a portion of the waveformaround a resonant ring;

FIG. 8 is a more detailed illustration of the resonant ring visible inFIG. 7 from which measurements can be made for computing impedanceparameters;

FIG. 9 is a graphical representation of an exemplary frequency domainplot of the ring illustrated in FIG. 8;

FIG. 10 is a diagrammatical representation of an incident energymeasurement system in accordance with aspects of the present technique;

FIG. 11 is a diagrammatical representation of an exemplary systememploying the incident energy measuring system;

FIG. 12 is a graphical representation of an exemplary display of PPElevels based upon determinations made via the systems of precedingfigures;

FIG. 13 is a diagrammatical representation of an exemplary MCCincorporating aspects of the present techniques;

FIG. 14 is a somewhat more detailed view of the exemplary MCC of FIG.13; and

FIG. 15 is a graphical representation of an exemplary portable incidentenergy measurement device, again incorporating aspects of the presenttechniques.

DETAILED DESCRIPTION

Turning now to the drawings, and referring first to FIG. 1, an impedancemonitoring system is illustrated and designated generally by thereference numeral 10. The impedance monitoring system is illustrated ina single-phase application. That is, the system is illustrated foridentifying the impedance of a single-phase power source. As will beappreciated by those skilled in the art, and as discussed in greaterdetail below, the system may be easily adapted for identifying impedanceparameters of three-phase power lines and sources as well.

Impedance monitoring system 10 is illustrated as coupled to a pair ofpower supply lines 12. Power supply lines 12, and any upstreamcircuitry, such as transformers, connectors, and so forth are consideredto have a net impedance illustrated by equivalent circuitry in box 14 ofFIG. 1. The impedance 14 is, for the present purposes, considered to bea collective or cumulative impedance of the entire power supply network,represented generally by reference numeral 16 to a point between a powersupply grid and a load 18. As discussed in greater detail below, thepresent system provides the potential for determining impedance bymeasurement at or adjacent to a load 18. In practical applications, themonitoring system 10 may be coupled at any point along the power supplylines.

Impedance 14 is generally considered to include inductive components 20and resistor components 22. The inductive and resistive components willbe present in both supply lines, although for the present purposes thesecomponents may be grouped or accumulated into a net inductive componentand a net resistive component as discussed in greater detail below.

System 10 includes line test circuitry 24 for perturbing the voltagewaveform transmitted through the power lines and for making measurementsof the voltage waveform. The line test circuitry 24 is coupled to andworks in conjunction with data processing circuitry 26. As discussed ingreater detail below, the line test circuitry 24 and the data processingcircuitry 26 may, in certain applications, be analog circuitry, or atleast partially comprise analog circuitry. In a present embodiment,however, the line test circuitry and the data processing circuitrydigitally sample voltage measurements and store the sampled data in amemory 28. The stored sampled voltage measurements are then analyzed todetermine parameters of the voltage waveform that are used to computethe values of inductive and resistive components of the line impedance.As will be apparent to those skilled in the art, the data processingcircuitry 26 and memory 28 may be any suitable form. For example, bothof these components may be included in a general purpose orapplication-specific computer. Moreover, the circuitry may be local andpermanently installed with an application, or may be portable circuitry,such as in hand-held devices. Similarly, the data processing circuitryand memory may be entirely remote from the line test circuitry so as toprovide the desired analysis without necessarily displacing equipment oroperators to the test site.

The data processing circuitry 26 may be accessed and interfaced withoperator workstations by interface circuitry 30. The interface circuitry30 may include any suitable interfaces, such as Ethernet cards andinterfaces, Internet access hardware and software, or other networkinterfaces. In appropriate situations, the interface circuitry 30 mayallow for interfacing with the data processing circuitry by conventionalserial port communication, and so forth. As illustrated in FIG. 1,various operator interfaces may be envisioned, including laptopcomputers, computer workstations, and so forth, as represented generallyby reference numeral 32 in FIG. 1.

The line test circuitry 24 is illustrated in somewhat greater detail inFIG. 2, along with the physical relationship between the portions of thecircuitry. As noted above, the collective or cumulative impedance in thepower lines may be diagrammatically represented as a single inductivecomponent 20 and a resistive component 22. The line test circuitry 24includes voltage perturbation circuitry 33. The voltage perturbationcircuitry 33 includes a resistor 34 which is coupled in series with acapacitor 36. A diode 35 and a solid state switch 38 are coupled inparallel with the capacitor 36 so as to permit the capacitor 36 to bebypassed by creating a short circuit between the power lines during atest sequence as summarized below. Those skilled in the art willappreciate that the diode 35 may be omitted in direct currentembodiments. Where desired, an enable switch, represented generally atreference numeral 40, may be provided in series with these components. Aswitch 40 may permit an operator to enable a test sequence, whileremoving the components from the power line circuit during normaloperation. Thus, switch 40 may permit any leakage current between thepower lines to be avoided.

Voltage measurement circuitry 42 is provided that spans the power lineconductors. The voltage measurement circuitry 42 may include anysuitable voltage measurement configurations, and is particularly adaptedto sample voltage across the power lines and to provide valuesrepresentative of the sampled voltage to data processing circuitry 44.The data processing circuitry 44 includes the data processing circuitry26 and the memory 28 illustrated in FIG. 1, along with any appropriateprogramming for carrying out the functions, measurements, and analysesdescribed below. To initiate and advance the test sequences formeasuring the parameters of the power line impedance, the dataprocessing circuitry 44 is coupled to driver circuitry 46 which providessignals to solid switch state 38 to open and close the switch asdescribed in greater detail below.

Although the present invention is not intended to be limited to anyparticular circuit configuration or component values, the followingcomponent values have been found effective in identifying impedanceparameters in a 60 Hz power source. Resistor 34 was implemented as a 1Ωresistor, while the value of capacitor 36 was 22 μF. The switch 38 wasselected as an insulated gate bipolar transistor (IGBT) having a voltagerating of 1200V and amperage rating of 400 A. It is advisable that theswitch 38 be overrated to some degree to permit peaks in the voltagewaveform that may result from opening and closing of the switch, andparticularly the affects of the resonant ring following closure.

Exemplary logic 48 for a particular test sequence implemented by thecircuitry of FIG. 2 is illustrated diagrammatically in FIG. 3. As notedabove, voltage test circuitry 42 first begins to sample voltage acrossthe power lines as indicated at reference numeral 50. At a desired pointin the waveform, the switch 38 is closed, as indicated at step 52 inFIG. 3. Closure of switch 38 (with switch 40 closed to enable thecircuitry, where such a switch is provided) causes a short circuitbetween the power lines, by routing current around capacitor 36. The lowvalue of the resistor 34 acts as a drain or burden, causing a droop inthe voltage waveform peak as described in greater detail below.Subsequently, switch 38 is opened, as indicated at reference numeral 54.Opening of the switch then causes a resonant ring between the inductivecomponent 20 of the line impedance and the capacitor 36. This resonantring is dampened by the resistive component 22 of the power lineimpedance and by the resistor 34.

With the voltage continuously being measured (i.e., sampled) by thevoltage measurement circuitry 42, measurements are stored in the memorycircuitry for later analysis. In a present implementation, with digitalsampling of the voltage waveform, at step 56 in FIG. 3, the voltage andring parameters used to identify the inductive and resistive componentsof the line impedance are then determined. At step 58 the inductive andresistive components of the line impedance are then computed based uponthese identified values.

Thus, with steps 50 through 58 being carried out, the system response ismeasured continuously through the sampled data. These measurements aresummarized at step 60 in FIG. 3, where a value of the voltage with theline test circuitry open is measured, and step 62 where a voltage acrossthe power lines with the resistor 34 in short circuit between the powerlines is measured. Step 64 represents measurement of the ring parametersused in the subsequent computations.

The calculations made of the inductive and resistive components of thepower line impedance in accordance with the present techniques may bebased upon the following computational scheme. As will be appreciated bythose skilled in the art, the inductive-capacitive (LC) resonantfrequency established upon opening of switch 38, with little or nodamping in the circuit may be expressed by the relationship:

$\begin{matrix}{{2\pi\; f} = \frac{1}{\sqrt{{LC}\;{load}}}} & {{Equation}\mspace{20mu} 1}\end{matrix}$where f is the resonant frequency of the LC circuit, L is the value ofthe inductive component of the line impedance, and the parameter Cloadis the value of the capacitor 36 discussed above.

It will be noted, however, the resistor 34, particularly where a verylow value of resistance is chosen, will provide significant damping tothe resonant ring. Indeed, as will be appreciated by those skilled inthe art, the value of the resistor 34 chosen strikes a balance betweenthe desire to adequately sample a voltage droop caused by the drainrepresented by the resistor, while providing a significantly long (i.e.,less damped) resonant ring to permit measurement of the ring period orfrequency. Considering such damping, the relationship indicated inEquation 1 becomes described by the following relationship:

$\begin{matrix}{{2\pi\; f} = \sqrt{\frac{1}{LCload} - \left( \frac{R + {Rload}}{2L} \right)^{2}}} & {{Equation}\mspace{20mu} 2}\end{matrix}$where the value R represents the value of the resistive component of theline impedance, and the value Rload represents the rating of theresistor 34 discussed above.

Based upon equation 2, and solving for the value of the line inductanceL, the following relationship may be expressed in terms only of thevalues of Cload, Rload and the frequency f:

$\begin{matrix}{L = \frac{\frac{1}{Cload} + \sqrt{\frac{1}{{Cload}^{2}} - {\left( {2\pi\; f} \right)^{2}{Rload}^{2}}}}{2\left( {2\pi\; f} \right)^{2}}} & {{Equation}\mspace{20mu} 3}\end{matrix}$

To complete the system of equations desired for calculating theinductive and resistive components of the line impedance, in accordancewith the present techniques, the voltage sag or droop caused by closureof switch 38 and the presence of the drain or burden resistor 34 may beexpressed in terms of the voltage sampled across the power lines withthe line test circuitry open, indicated by the quantity Vo, and thevoltage across the power lines with the circuitry closed, Vr, that is,with the resistor 34 in a series across the power lines as follows:

$\begin{matrix}{{Vr} = {{Vo}\frac{Rload}{{j\; 337L} + R + {Rload}}}} & {{Equation}\mspace{20mu} 4}\end{matrix}$where Vo and Vr are either the peak or RMS ac voltage values. It shouldbe noted that the value 377 in Equation 4 (and in the subsequentequations below) is derived from the product of 2π and a line frequencyof 60 Hz. As will be appreciated by those skilled in the art, theequations will differ for other line frequencies, although theprinciples for computation of the line impedance parameters will remainunchanged. Equation 4 may be rewritten as follows:

$\begin{matrix}{{Vr} = {{Vo}\frac{Rload}{\sqrt{\left( {377L} \right)^{2} + \left( {R + {Rload}} \right)^{2}}}}} & {{Equation}\mspace{20mu} 5}\end{matrix}$

It may be noted that Equation 5 may be solved in terms of the value ofthe resistive component of the line impedance, R, as follows:

$\begin{matrix}{R = {\sqrt{\frac{\left( {{Vo}\mspace{11mu}{Rload}} \right)^{2} - \left( {{Vr}\; 377L} \right)^{2}}{{Vr}^{2}}} - {Rload}}} & {{Equation}\mspace{20mu} 6}\end{matrix}$

Thus, based upon three measured values alone, the values of theinductive component of the line impedance, L, and the resistivecomponent of the power line impedance, R, may be computed by Equations 3and 6. The measured values, in accordance with the present technique,are the values of Vo, Vr, and the frequency f, or the period, which willbe appreciated by those skilled in the art, is the inverse of thisfrequency value.

FIG. 4 illustrates an exemplary ac voltage waveform and a switchingwaveform for the switch 38 during an exemplary test sequence inaccordance with FIG. 3 to measure values for use in calculating theimpedance parameters in accordance with Equations 3 and 6 discussedabove. FIG. 4 illustrates the waveforms graphically as indicatedgenerally by reference numeral 56. The voltage waveform is illustratedgraphically with respect to voltage, as indicated axis 68 over time, asindicated by axis 70. The voltage trace 72 of the waveform takes theform of a sine wave. Trace 74 represents the state of switch 38 or, moreparticularly, the signal applied to drive the gate of the switch tochange its conductive state during the testing procedure.

As can be seen from FIG. 4, once sampling of the waveform has begun,samples will be taken continuously at a desired frequency, above theNyquist rate, to provide reliable data for analysis. In a first cycle 76of the waveform, with the test circuit open, a peak voltage 78,corresponding to Vo will be detected, among the other values detectedand stored. At some point during or preceding a second cycle 82, switch38 is placed in a conductive state to complete the current carrying pathbetween the line conductors. The change in state of the switch isindicated at the rising edge 74 of the waveform, and occurs at time 80.As a result of the significant voltage drain caused by resistor 34, asag or droop is detected in the peak 84 of the voltage waveform, withthe peak generally corresponding to the value Vr. Subsequently, theswitch 38 is opened, as indicated by the drop in waveform 74 that occursat time 86 indicated in FIG. 4. The opening of switch 38 causes aresonant ring as indicated generally at reference numeral 88. As notedabove, the resonant ring may have a peak voltage above the peak voltageof the sinusoidal waveform, and the switch 38 may be selected toaccommodate such peaks.

FIG. 5 illustrates a more detailed view of the resonant ring occurringfrom opening of the switch of the line test circuitry. Again graphedwith respect to a voltage axis 68 and a time axis 70, the ring occurs asthe voltage across the lines is decreasing, as indicated by the initialslope of trace 72. The falling edge of waveform 74 represents theremoval of the drive signal to the switch causing opening of thecircuit. The resulting resonant ring 88 will have a period, orconsequently a frequency, that is a function of the circuit componentparameters and of the inductive component of the line impedance. Becausethe voltage waveform is continuously sampled, the frequency or periodmay be measured, with a full period being indicated by reference numeral90 in FIG. 5. As will be apparent to those skilled in the art, theperiod may be measured in a number of ways, as may the frequency. Forexample, a half cycle alone may be used to determine the frequency orperiod, or a full or even more than one cycle may be used. Similarly, ina present embodiment, the values of the ring as sampled by the circuitrymay be compared or processed with the naturally declining value of thesinusoidal waveform to provide an accurate indication of the period ofthe resonant ring. Based upon the measured voltages, Vo, Vr and eitherthe period of the resonant ring or its frequency, then, Equations 3 and6 may be employed or determining the values of L and R.

An alternative approach to identifying the parameters discussed above isillustrated in FIGS. 6–9. As illustrated in FIG. 6, the voltage waveformthat is sampled by the voltage measurement circuitry may be illustratedas having successive cycles 76 and 82, with a voltage droop or sagoccurring in cycle 82 due to the resistor 34 discussed above. Thesubsequent ring upon a removal of the short circuit across the powerlines is again indicated at reference numeral 88. The data may behigh-pass filtered to generally flatten the waveform as indicated atreference numeral 92 in FIG. 7. The high-pass filtered waveform willthen exhibit the ring which may be timed to occur during a generallylinear portion of the sine wave, as indicated at reference numeral 94 inFIG. 7. From the data, the ring 94 may be extracted as indicatedgenerally in FIG. 8. The period, or half period, or frequency of thering may then be determined, as indicated at reference numeral 90 inFIG. 8. Finally, where desired, the waveform may be converted by aone-dimensional fast Fourier transform to a frequency responserelationship as indicated in FIG. 9. This frequency response may berepresented graphically along an amplitude axis 98 and a frequency axis100. The frequency trace 102 in FIG. 9 indicates a resonant frequency atpeak 104 which generally corresponds to the wavelength measured for theresonant ring as discussed above. As noted, either the frequency or theperiod of the waveform may be used for the calculation of the impedanceparameters.

FIG. 10 is a diagrammatical representation of an incident energymeasurement system 106 in accordance with aspects of the presenttechnique. The incident energy measurement system 106 comprises modulesrepresented by blocks 10, 108, 110, 112, 114, 116, and 118. The modules(blocks 10 and 108–118) may be hardware, software, firmware, or somecombination of hardware, software, and firmware. Additionally, anindividual module does not necessarily solely comprise each illustratedmodule function. The modules shown in the blocks 10 and 108–118 aremerely one example and other examples can be envisaged wherein thefunctions are distributed differently or where some modules are includedand other modules are not included. Further, FIG. 10 also illustratesinputs 120 and outputs 122 from each of the modules 10 and 108–118.Those of ordinary skill in the art will appreciate that the inputs 120and the outputs 122 are exemplary. In alternate embodiments, the inputs120 and the outputs 122 to each of the modules 10 and 108–118 maydiffer. Similarly, while FIG. 10 call for certain “inputs,” in manyinstallations, the values used in the various determinations will beknown in advance, may be pre-programmed in the modules, or may beprovided in a menu for user selection. Lastly, while the incident energymeasurement system 106 is described herein in regard to a three phasesystem, those of ordinary skill in the art will appreciate that thetechniques herein may be applied to single phase or dual phase systems.

As described above, the impedance monitoring module 10 (previouslyreferred to as the impedance monitoring system 10) may receive orcalculate a Cload value 124 (e.g., the value of the capacitor 36,described above), and an Rload value 126 (e.g., the rating of theresistor 34, described above), a resonant ring frequency f 128, avoltage Vo 129 (the voltage sampled across the power line with the linetest circuitry opened), and a voltage Vr 132 (a voltage across the powerline with the circuitry closed) for each phase of a three phase powertransmission system. From these inputs, as described in relation toEquations 1–6 above, the impedance monitoring module 10 may compute aninductive component of the line impedance (L) 134 and a resistivecomponent of the line impedance (R) 136. As illustrated in FIG. 10, theinductive component 134 and the resistive component 136 may be eithercommunicated by the system 106 as outputs, transmitted to the boltedfault current calculation module 108, or both

The bolted fault current calculation module 108 may calculate a boltedfault current (Ibf) 138 using Ohm's law for AC circuits, which is:

$\begin{matrix}{{Ibf} = \frac{V}{Z}} & {{Equation}\mspace{20mu} 7}\end{matrix}$where Ibf is the bolted fault current 138, V is the three phase systemvoltage Vs 130 calculated based on the voltage Vo 129 for each of thethree phases, and Z is a line impedance calculated from the inductivecomponent 134 and the resistive component 136 for each of the threephases. As illustrated in FIG. 10, the bolted fault current 138 mayeither be communicated by the system 106, transmitted to the arc currentcalculation module 110, or both

The arc current calculation module 110 calculates an arc current Ia 144.In one embodiment, the arc current calculation module 110 calculates thearc current Ia 144 based on the equations set forth in the Institute forElectrical and Electronics Engineers (“IEEE”) Guide for PerformingArc-Flash Hazard Calculations, IEEE Std. 1584 (2002), which is herebyincorporated by reference. Specifically, if the voltage Vo is less than1000 volts, the arc current 144 may be calculated as follows:log(Ia)=K1+0.662log(Ibf)+0.966V+0.000526G+0.5588V(log(Ibf)−0.00304G(log(Ibf))  Equation8where the arc current 144 equals 10^(log(Ia)); K1 equals −0.153 for openconfigurations (i.e., configurations where the conductors that may arcare not contained within a chassis or enclosure) and −0.097 for enclosedconfigurations; V is the voltage Vs 130 in kilovolts (KV), G is a gap142 between the conductors that could potential arc in millimeters (mm);and Ibf is the bolted fault current 138 in kiloamps (KA). If the voltageV is greater than 1000 volts, the following equation is used instead ofEquation 8:log(Ia)=0.00402+0.983 log(Ibf).  Equation 9

Those of ordinary skill in the art will appreciate that Equations 8 and9 are derived empirically using statistical analysis and curve fittingprograms. As such, they are only applicable for voltages Vs 130 in therange of 208V–15,000V, power line frequencies of either 50 Hz or 60 Hz,bolted fault currents 138 in the range of 700 A–106,000 A, and for gapsbetween conductors 142 of 13 mm to 152 mm. In embodiments incorporatingparameters outside these ranges, the theoretically-derived Lee's methodcan be used in place of the modules 110 and 112, as will describedfurther below.

Once calculated, the arc current 144 may either be communicated by thesystem 106, transmitted to the normalized incident energy calculationmodule 112, or both. In one embodiment, the normalized incident energycalculation module 112 calculates a normalized incident energy 148 usingthe following equation:log(En)=K2+K3+1.081 log(Ia)+0.0011G  Equation 10where 10^(log(En)) is the normalized incident energy 148 in joules persquare centimeter (J/cm²); K2 is −0.792 for open configurations and−0.555 for enclosed configurations; K3 is zero for ungrounded orhigh-resistance grounded systems and −0.113 for grounded systems; Ia isthe arc current 144; and G is the gap 142. Equation 10 calculates theincident energy normalized for an arc time t of 0.2 seconds and adistance from the possible arc point (e.g., the MCC) to a measurementpoint of 610 mm. One of ordinary skill in the art will appreciate thatin alternate embodiments, the normalized incident energy calculationmodule 112 may employ an alternate version of Equation 10 that has beennormalized for a different arc time t or a different distance D.Further, as described above, for embodiments where one of the parametersVo, f, Ibf, or G falls outside of the empirically tested range (seeabove), the normalized incident energy calculation module 112 may beabsent from incident energy measurement system 106.

Once calculated, the normalized incident energy 148 may either becommunicated by the system 106, transmitted to the incident energycalculation module 114, or both. Unlike the normalized incident energycalculation module 112 which is normalized for a distance of 610 mm, theincident energy calculation module 114 is configured to calculate anincident energy 156 in J/cm² at an arbitrary distance D 152 from thepoint of the potential arc. For example, the incident energy calculationmodule 114 can calculate the incident energy 156 at one meter from anMCC or three meters, and so forth. In one embodiment, the incidentenergy calculation module 114 calculates the incident energy 156 withthe following equation:

$\begin{matrix}{E = {4.184({Cf})({En})\left( \frac{t}{0.2} \right)\left( \frac{610^{x}}{D^{x}} \right)}} & {{Equation}\mspace{20mu} 11}\end{matrix}$where E is the incident energy 148; Cf is 1.0 if the voltage Vs 130 isgreater than 1000 volts and 1.5 if the voltage Vs 130 is equal to orless than 1000 volts; En is the normalized incident energy 148; t is anarc time 150, which is described in greater detail below; D is thedistance 152; and x is a distance exponent, which is also described ingreater detail below.

As noted above, one of the elements in Equation 11 is the arc time 150.The arc time 150 is a factor in calculating the incident energy 156,because the amount of energy generated by the arc flash is proportionalto the length of time that the arc current 144 is actually flowing(i.e., the device is arcing). Those of ordinary skill in the art willappreciate that in most situations devices such as circuit breakers orfuses will detect the sudden increase in current that accompanies an arcflash and interrupt power to the system. Unfortunately, these devicesare not instantaneous, and the arc current 144 will flow forapproximately as long as it takes the devices to activate and interruptthe power. Equation 10 above (the normalized incident energycalculation) is normalized for an arc time of 0.2 seconds (i.e., the arccurrent 144 flows for 0.2 seconds). However, the arc time 150 for aparticular system may not be 0.2 seconds. As such, one of the inputs tothe incident energy calculation module 114 may be the arc time 150 forthe system of interest.

Those of ordinary skill in the art will appreciate that there areseveral techniques for determining the arc time 150. For example, themanufacturer of an electrical system, such as a circuit breaker, a fuse,or a circuit interrupter, may provide the arc time 150 for theelectrical system. Also, IEEE Std. 1584 provides versions of Equation 11pre-calculated with the arc times 150 for a variety of standard types offuses or circuit breakers. Further, the arc time 150 may also bedetermined by charting the time/current characteristics for the circuitbreaker or fuse that will sever the electrical connection and stop theflow of the arc current 144.

As described above, Equation 11 also employs the distance exponent x.The distance exponent x may be determined using a look-up table, such asTable 1 (below) from IEEE Std. 1584 where an equipment type 154 (openair, switchgear, MCC and panels, or cable) is entered in the incidentenergy calculation module 114 as an input.

TABLE 1 Vs Equipment Type Distance Exponent x  208 V–1000 V Open Air 2.0Switchgear 1.473 MCC and panels 1.641 Cable 2.0 1000 V–5000 V Open Air2.0 Switchgear 0.973 Cable 2.0   5000 V–15,000 V Open Air 2.0 Switchgear0.973 Cable 2.0In one embodiment, the incident energy calculation module 114 isprogrammed with a look-up table (LUT) comprising Table 1.

In an alternate embodiment of the incident energy measurement system106, the incident energy calculation module 114 calculates the incidentenergy using Lee's method. As stated earlier, Lee's method istheoretical and, thus, can be used to calculate the incident energy 156outside the empirical range of Equations 8–11; (i.e., Vs greater than15,000V, arc fault currents greater than 106,000 A, gaps betweenconductors larger than 152 mm, and so forth). As Lee's method is basedon the arc fault current 138, the modules 112 and 114 may be omittedfrom an embodiment of the incident energy measurement system 106 thatemploys Lee's method. The incident energy calculation module 114 can usethe following equation to calculate the incident energy 156 using Lee'smethod:

$\begin{matrix}{E = {2.142*10^{6}(V)({Ibf})\left( \frac{t}{D^{2}} \right)}} & {{Equation}\mspace{20mu} 12}\end{matrix}$where E is the incident energy 156 measured in J/cm²; V is the voltageVs 130; t is the arc time 150 in seconds; D is the distance 152; and Ibfis the arc fault current 138. Once the incident energy calculationmodule 114 has calculated the incident energy 156 at the distance 152this value may be reported by the incident energy measurement system106.

As illustrated in FIG. 10, the incident energy measurement system 106may also comprise the flash protection boundary calculation module 116.The flash boundary calculation module 116 may operate in conjunctionwith or in alternative to the incident energy calculation module 114. Asits title suggests, the flash protection boundary calculation module 116calculates a flash protection boundary 160 for the system being measured(e.g., for an MCC). Those of ordinary skill in the art will appreciatethat the incident energy decreases proportionally as the distance fromthe arcing point increases. At some distance from the origination pointof the arc, the incident energy is low enough to be consideredacceptable. This distance is known as the flash protection boundary 160.In one embodiment, the flash protection boundary 160 is deemed to existat a distance from the arcing point where an incident energy Eb 158 isequal to 5.0 J/cm².

The flash protection boundary calculation module 116 is similar to theincident energy calculation module 114 except that rather thandetermining the incident energy 156 at the distance 152, the flashprotection boundary calculation module 116 determines the flashprotection boundary (i.e., a distance) where the incident energy 156will be at the incident energy level Eb 158. In one embodiment, theflash protection boundary calculation module 116 employs the followingequation:

$\begin{matrix}{{Db} = {4.184({Cf})({En})\left( \frac{t}{0.2} \right)\left( \frac{610^{x}}{{Eb}^{x}} \right)^{\frac{1}{x}}}} & {{Equation}\mspace{20mu} 13}\end{matrix}$where Db is the flash protection boundary 160 in millimeters; Cf is 1.0if the voltage Vs 130 is above 1000 volts and 1.5 if the voltage Vs 130is equal to or less than 1000 volts; t is the arc time 150; Eb is theincident energy 158 at the flash protection boundary 160 (e.g., 5.0J/cm²); and x is the distance exponent, as described above. Oncecalculated, the flash protection boundary 160 can be reported by theincident energy measurement system 106.

The flash protection boundary 160 can also be determined using Lee'smethod. Specifically, the flash protection boundary calculation module116 may employ the following equation:

$\begin{matrix}{{Db} = \sqrt{2.142*10^{6}(V)({Ibf})\left( \frac{t}{{Eb}^{2}} \right)}} & {{Equation}\mspace{14mu} 14}\end{matrix}$where Db is the flash protection boundary 160; V is the voltage Vs 130;Ibf is the arc fault current 138; t is the arc time 150; and Eb is theincident energy 158.

Alternatively or in conjunction with the incident energy calculationmodule 114 and the flash protection boundary calculation module 116, theincident energy measurement system 106 may also include a PPE levelcalculation module 118. In one embodiment, the PPE level calculationmodule 118 calculates the PPE level 161 at one or more distances 152from the potential arcing point. For example, the PPE level calculationmodule 114 may calculate that level 1 PPE is appropriate at six metersfrom the potential arc point or that level 3 PPE is appropriate for workbeing performed on equipment in the same MMC as the potential arcingpoint. To determine the PPE level 161, the PPE level calculation module118 may employ either Equation 11 or 12, as outlined above, inconjunction with the following table from NFPA 70E, which is herebyincorporated by reference:

TABLE 2 PPE Category Eb (in J/cm²) 0 <5.0 1  5.0–16.74 2 16.74–33.47 333.47–104.6 4  104.6–167.36

For example, the PPE level calculation module 118 may compute anincident energy of 18.24 J/cm² at 0.1 meters using Equation 11, asdescribed above. Because 18.24 J/cm² falls between 16.74 and 33.47, thePPE level calculation module 118 would determine that level 2 PPE isappropriate at 0.1 meters from the arc point. Once the PPE levelcalculation module 118 determines the PPE level 160, it may communicatethis determination out of the incident energy measurement system 106 fordisplay, as further described below. In one embodiment, Table 2 isstored in the PPE level calculation module 118 as a look-up table.

FIG. 11 is a diagrammatical representation of an exemplary system 162employing the incident energy measuring system 106. The system 162comprises a three phase power bus 164 and a data bus 166. The threephase power bus may be coupled to the power supply grid 16. Asillustrated, the three phase power bus 164 provides three phase power tobus bars 165 within MCCs 170 and 172. This form of power distribution iswell known to those of ordinary skill in the art and need not bedescribed in greater detail. The data bus 166 provides a communicationpathway between a remote command and control unit 168 and the MCCs 170and 172. As will be described in greater detail below, the illustratedsystem 162 includes two types of exemplary MCCs 170 and 172. The MCC 170comprises a distributed incident energy monitoring system 106, while theMCC 172 comprises a stand-alone incident energy monitoring system 106.Those of ordinary skill in the art will appreciate that the MCCs 170 and172 are exemplary. In alternate embodiments, virtually any type ofelectrical device or apparatus suitable for use with the incident energymonitoring system 106 may employ the incident energy monitoring system106 in the manner described below.

The control unit 168 may comprise a network interface 174, incidentenergy calculation circuitry 176, memory 178, and a computer 180. Thenetwork interface 174 facilitates communication between a control centeror remote monitoring station and the MCCs 170 and 172. In oneembodiment, the control unit 168 communicates with the voltagemeasurement circuitry 42 disposed on the MCC 170 via the networkinterface 174. The incident energy calculation circuitry 176 comprisesthe data processing circuitry 44 and some or all of the modules 108–118.The incident energy calculation circuitry 176 receives the inputs 120(see FIG. 10) from the voltage measurement circuit 42 disposed in theMCC 170 or from a memory 178, which is programmed with some or all ofthe inputs 120. The incident energy calculation circuitry 176 employsthe inputs 120 to produce the outputs 122, as described above withregard to FIGS. 2 and 10. In one embodiment, the control unit 168 isconfigured to trigger the measurement of the incident energy 156 or anyof the other outputs 122. In another embodiment, the outputs 122, oncecalculated, are transmitted to the computer 180 within the control unit168. In still another embodiment, the computer 180 may be employed toprogram the memory 178 with the inputs 120.

As stated above, the MCC 170 includes a distributed version of theincident energy monitoring system 106. In this embodiment, the MCC 170works in conjunction with the control unit 168 to determine the outputs122, as described above. As such, the MCC 170 comprises only a portionof the incident energy monitoring system 106—namely, the voltagemeasurement circuitry 42 and the voltage perturbation circuitry 33. Thevoltage perturbation circuitry 33 and the voltage measurement circuitryfunction as described above in regard to FIG. 2 except that the voltageperturbation circuitry 33 and the voltage measurement circuitrycommunicate with the data processing circuitry 44 within the incidentenergy calculation circuitry 176 via a network interface 182 and thedata bus 166. In other words, circuitry within the MCC 170 is configuredgenerate the resonant ring 88 (see FIG. 4), to measure the resonant ring88, and to communicate the measurements to the incident energycalculation circuitry 176, which determines the outputs 122. In oneembodiment, the control unit 168 may transmit a portion of the outputs122 back to the MCC 170 for display on a display 184, as will bedescribed further below in regard to FIG. 12. Those skilled in the artwill appreciate that in alternate embodiments, the system 162 maycomprise multiple MCCs 170 each of which is supported by a singlecontrol unit 168. Further, in still other embodiments, some of themodules 108–118 may be located in the MCC 170 instead of the controlunit 168. Moreover, in yet other embodiments, the MCC 170 may beconfigured to process the voltage measurements from the voltagemeasurement circuitry prior to communicating with the data processingcircuitry 44.

Turning next to the stand-alone MCC 172, the MCC 172 comprises theincident energy monitoring system 106. As illustrated, the incidentenergy measurement system 106 is coupled to the three phase power bus166 via the bus bars 165. As such, the impedance monitoring module 10within the incident energy monitoring system 106 can function asdescribed above in regard to FIGS. 1–10. The MCC 172 also comprises thememory 178, which can provide the inputs 120 to the incident energymeasurement system 106. The memory 178 may either be programmed by acomputer coupled directly to the MCC 172 (not shown) or by the computer180 in the control unit 168 via the data bus 166 and a network interface186. The incident energy monitoring system 106 within the MCC 172 may beconfigured to display one or more of the outputs 122 on the display 184,as described further below. In addition, in one embodiment, the MCC 172is configured to transmit one of more of the outputs 122 to the controlunit 168 via the data bus 166.

Those skilled in the art will appreciate that the data bus 166 and thenetwork interfaces 174, 182, and 186 may employ a wide variety ofsuitable communication technologies or protocols. For example, in oneembodiment, the data bus 166 may comprise a local area network, and thenetwork interfaces 174, 182, and 186 may comprise network interfacecards. In yet another example, the data bus 166 may comprise a wirelessnetwork based on the IEEE 802.11 standard or another suitable wirelesscommunication protocol, and the network interfaces 174, 182, and 186 maycomprise wireless transmitters and receivers.

FIG. 12 is a graphical representation of an exemplary display 184 of PPElevels based upon determinations made via the systems of precedingfigures. Both the MCC 170 and the MCC 172 may include the display 184 todisplay one or more of the outputs 122 to a user or operator. Theillustrated display 184 comprises a set of PPE level indicator lights188. In one embodiment, the display 184 may be mounted in a front panelof the MMC 170 or 172. The PPE level indicator lights 188 may beconfigured to display the level of PPE appropriate for performingmaintenance on the MMC 170 or 172. Specifically, a particular lightcorresponding to the PPE level may be illuminated. In an alternateembodiment, a light tower or siren-style light mounted to the MCC 170 or172 may produce colored light indicative of the PPE level. The display184 may also comprise a screen 190. In one embodiment, the screen 190comprises a liquid crystal diode (LCD) display or other form of textualor graphical display. The screen 190 is configurable to display any ofthe outputs 122. For example, the screen 190 may be configured todisplay the flash protection boundary 160 or the normalized incidentenergy 148. As will be appreciated by those skilled in the art, thedetermination of the PPE level, and its display at or near the point ofentry for maintenance can greatly facilitate the task of donning thecorrect PPE prior to servicing of the equipment.

FIG. 13 is a diagrammatical representation of an exemplary MCC 172incorporating aspects of the present techniques. The MCC 172 comprises achassis 192, an incident energy measurement bucket 194, and a pluralityof motor control buckets 196. As described above, the MCC 172 comprisesthe bus bars 165, which are coupled to the three phase power bus 164.While not illustrated in FIG. 13, the data bus 166 may also be connectedto the MCC 172. Similarly, as will be appreciated by those skilled inthe art, in practice, the bus bars 165 may be disposed behind one ormore plates or barrier

The incident energy measurement bucket 194 comprises the incident energymeasurement system 106. Those of ordinary skill in the art willappreciate that there is an intrinsic resistance in the connectionbetween the voltage perturbation circuit 33 and the bus bars 165. Thisintrinsic resistance can affect the accuracy of the measurements of thevoltage measurement circuitry 42. To reduce the effects of thisintrinsic resistance, the voltage measurement circuitry 42 may becoupled to a first set of stabs 198 a and the voltage perturbationcircuitry 33 may be coupled to a second, separate set of stabs 198 b.Those of ordinary skill in the art will appreciate that coupling thevoltage measurement circuitry 42 to a different set of stabs than thevoltage perturbation circuitry 33 increases the accuracy of themeasurement of the voltage measurement circuit 42. That is, due to thesignificant current draw of the voltage perturbation circuitry 33,voltages that would be measured at that circuit could be significantlyaffected by the resistance of that circuit's stabs, fuses, and so forth.On the other hand, the current draw of the voltage measurement circuitry42 is negligible. The parallel connection of the two circuits, then,allows for more accurate measurements of the voltages during tests.

FIG. 14 is a somewhat more detailed view of the exemplary MCC 172. Forsimplicity, like reference numerals have been used to designate featurespreviously described in reference to FIG. 13. In the embodimentillustrated in FIG. 14, the first set of stabs 198 a are configured tobe coupled to the bus bars 165 at a location above (i.e., electricallycloser to the three phase power bus 164). While not illustrated in FIG.13 or 14, the incident energy measurement bucket may also comprise thedisplay 184, the memory 178, and the network interface 186, asillustrated in FIG. 11.

FIG. 15 is a graphical representation of an exemplary portable incidentenergy measurement device 200, again incorporating aspects of thepresent techniques. In one embodiment, the portable device 200 comprisesa laptop, tablet, or a portable computer system. In another embodiment,the portable device 200 comprises a personal digital assistant orpalm-top computer system. The portable device 200 comprises the incidentenergy measurement system 106 (not shown) and a plurality of test leads202 a, 202 b, and 202 c. The test leads 202 a–c are configured to beconnected to a power source, such as the three phase power bus 164. Inthe illustrated embodiment, the test leads 202 a–c comprise clips toconnect the test leads to the three phase power bus 164. In alternateembodiments, the test leads 202 a–c may comprise any form of connectorsuitable for connecting the test leads 202 a–c to a power source.

In the embodiment illustrated in FIG. 15, each of the test leads 202 a–care coupled to both the voltage perturbation circuitry 33 and thevoltage measurement circuitry 42, described above. In alternateembodiments, the portable device may comprise an additional set of testleads 202 a–c that are connected to the voltage measurement circuitry 42to increase the accuracy of the portable device 200, as described abovein regard to FIGS. 13 and 14. The portable device 200 may also contain apower source, such as a battery or an ac plug (not shown).

The portable device may also comprise a display 204 and an input device206. In one embodiment, the display 204 is a liquid crystal diode (LCD)display. The input device 206 may be an internal keyboard, an externalkeyboard, or a touch screen. In one embodiment, the display 204 and theinput device 206 comprise a single touch screen. The portable device 200may also comprise a communication interface 208 for connecting theportable device 200 to a computer system. The communication interface208 may employ any communication protocol suitable for communicationbetween the portable device 200 and the computer. For example, thecommunication interface 208 may comprise a USB port, a Firewire port, anEthernet port, a Bluetooth transmitter and receiver, an 802.11transmitter and receiver, and so forth.

In operation, the operator of the portable device 200 connects the testleads 202 a–c to each of the phases of the three phase power bus 164.Because the portable device 200 is preprogrammed with the Cload value124 and the Rload value 126, the impedance monitor module 10 (not shown)within the portable device 200 can compute the inductive component ofthe line impedance 134 and the resistive component of the line impedance136 for each of the phases of the three phase power bus 164, asdescribed above in regard to FIG. 1–9. These values (134 and 136) canthen be displayed on the display 204.

In addition, the incident energy measurement system 106 within theportable device 200 can also determine the bolted fault current 138, thearc current 144, the normalized incident energy 148, the incident energy156, the flash protection boundary 160, and/or the PPE level 161, asdescribed above in reference to FIG. 10. In one embodiment, the inputs120 (FIG. 10) may be entered into the device 200 via the input device206. In another embodiment, the inputs 120 may be stored on a memory(not shown) within the portable device 200. In either case, the incidentenergy measurement system 106 within the portable device 200 determinesone or more of the outputs 122 and then transmits the outputs 122 to thedisplay 204.

Many of the modules described above with reference to FIG. 10 maycomprise an ordered listing of executable instructions for implementinglogical functions. These ordered listing can be embodied in acomputer-readable medium for use by or in connection with acomputer-based system that can retrieve the instructions and executethem to carry out the previously described processes. In the context ofthis application, the computer-readable medium can be a means that cancontain, store, communicate, propagate, transmit or transport theinstructions. By way of example, the computer readable medium can be anelectronic, a magnetic, an optical, an electromagnetic, or an infraredsystem, apparatus, or device. An illustrative, but non-exhaustive listof computer-readable mediums can include an electrical connection(electronic) having one or more wires, a portable computer diskette, arandom access memory (RAM) a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM or Flash memory), an optical fiber,and a portable compact disk read-only memory (CDROM). It is evenpossible to use paper or another suitable medium upon which theinstructions are printed. For instance, the instructions can beelectronically captured via optical scanning of the paper or othermedium, then compiled, interpreted or otherwise processed in a suitablemanner if necessary, and then stored in a computer memory.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A system for determining power line impedance comprising: a pluralityof bus bars coupled to a power source; and a first set of connectionpoints electrically coupled to the bus bars; a second set of connectionpoints electrically coupled to the bus bars; a voltage perturbationcircuit coupled to the plurality of bus bars through the first set ofconnection points and configured to generate a resonant ring; a voltagemeasurement circuit coupled to the plurality of bus bars through thesecond set of connection points and configured to measure a frequency ofthe resonant ring; and a processing circuit for determining inductiveand resistive components of an impedance of the power line based on thefrequency of the resonant ring.
 2. The system of claim 1, furthercomprising an incident energy calculation module for calculating anincident energy based at least partially on the impedance and aplurality of inputs comprising system parameters stored in a memory. 3.The system of claim 2, comprising an input device configured to storethe plurality of inputs comprising system parameters in the memory. 4.The system of claim 1, further comprising a display panel for displayingone of more power line parameters.
 5. The system of claim 1, wherein theprocessing circuit is remote from the voltage perturbation circuit andthe voltage measurement circuit.
 6. A method comprising: measuring afirst voltage of an ac waveform applied to an electrical line over afirst electrical contact; draining current from the electrical line overa second electrical contact; measuring a second voltage of the acwaveform over the first electrical contact during a droop in voltageresulting from the current drain; removing the current drain to cause aresonant ring in the voltage in the electrical line; measuring afrequency of the resonant ring over the first electrical contact; andcomputing the line impedance based upon the measured first and secondvoltages and the frequency of the resonant ring.
 7. The method of claim6, comprising computing inductive and resistive components of the lineimpedance.
 8. The method of claim 6, comprising periodically samplingvoltage during the measuring steps and storing values representativethereof.
 9. The method of claim 8, wherein measuring the voltagesincludes measuring peak voltages based upon the stored sampled voltagevalues.
 10. The method of claim 6, wherein the electrical line carriessingle phase power.
 11. The method of claim 6, comprising computing anincident energy based on the line impedance.
 12. The method of claim 11,comprising displaying the incident energy.
 13. The method of claim 6,comprising computing a flash protection boundary based on the lineimpedance.
 14. A method comprising: coupling a first circuit to avoltage source via a first electrical contact, wherein the first circuitis configured to perturb the ac waveform of the voltage source; couplinga second circuit to the voltage source via a second electrical contact,wherein the second circuit is configured to measure characteristics ofthe ac waveform and determine a line impedance of the voltage sourcebased on the characteristics of the ac waveform; and initiating aroutine to determine the line impedance.
 15. The method of claim 14,comprising initiating a routine to determine a level of personalprotective equipment, wherein the routine to determine the level ofpersonal protective equipment employs the line impedance.
 16. The methodof claim 15, comprising displaying indicia representative of thepersonal protective equipment associated with the determined level ofpersonal protective equipment.
 17. A method comprising: perturbing avoltage waveform through a first connection; measuring a characteristicof the perturbation through a second connection; and calculating a lineimpedance based on the characteristic of the perturbation.
 18. Themethod of claim 17, wherein measuring the characteristic comprisesmeasuring a frequency of a resonant ring.
 19. The method of claim 17,wherein measuring the characteristic comprises measuring acharacteristic for each phase of a three phase ac waveform.
 20. Themethod of claim 17, comprising determining an incident energy using theline impedance.
 21. The method of claim 17, comprising determining a PPElevel using the line impedance.
 22. A motor control center comprising: aplurality of bus bars coupled to a power source; and a first set ofstabs electrically coupled to the bus bars; a second set of stabselectrically coupled to the bus bars; a voltage perturbation circuitcoupled to the plurality of bus bars through the first set of stabs andconfigured to generate a resonant ring; and a voltage measurementcircuit coupled to the plurality of bus bars through the second set ofstabs and configured to measure a frequency of the resonant ring. 23.The motor control center of claim 22, further comprising a processingcircuit for determining inductive and resistive components of animpedance of the power line based on the frequency of the resonant ring.24. The motor control center of claim 22, further comprising an incidentenergy calculation module for calculating an incident energy based atleast partially on the impedance.
 25. The motor control center of claim22, wherein the motor control center is configured to communicate thefrequency of the resonant ring to a remote monitoring device.